coax/BNC issue

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gfdrob

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I recently installed a hole mount antenna on my Ford escape. Installed "c" mount per instructions. When attaching BNC crimp to coax end, if the outer braided wire touches the outer portion of the connector, I get no reception. If just the inner braided core wire is inserted into female antenna connection on scanner, I get decent reception. What am I doing wrong?
 

jackj

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It sounds like the center conductor is shorted to the braid. Go over the instructions and make sure you've got it installed right.
 

gfdrob

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Ok, I used a meter and determined there is no short from center conductor to braid. I do show continuity when I check braid to metal on vehicle and antenna rod to center conductor. When hooked to my GRE-500 the reception is terrible. I hooked same set up to a different scanner, reception is a BIT better but nothing like when I was using a mag mount. The reception was great, I was just tired of hearing the whistling of the coax going through the weatherstripping on the door, and could see the bends it was putting in the coax. I drilled my hole (3/8") in the center of the roof (same place I had the mag mount). Anybody have any suggestions? I'm about ready to put my mag mount back on, but need to find a 3/8" cap with a grommet in it to route my coax through.
Rob
 

jackj

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What is it about the reception that is terrible? Is there a whine or a pop/crackling that varies with engine speed? What kind of coax are you using? When I went back and re-read you original post, it sounded like you are using a double shielded coax of some sort.
 

gfdrob

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By terrible I mean that I am not picking up the range of stations I once did. It reminds me of the reception you would get from using a rubber duck antenna inside the car. This is at a "engine off" state so that is not an issue. I used a Comtelco CTM-02 mount, it is utilizing Teflon coax. The spec chart does not indicate that it is double shielded. The antenna mast itself picked up great when on the mag mount.

Just to put it into further perspective, when using the mag mount (same antenna mast) on my PSR-500, I could pick up 5 NOAA stations. Now I can only pick up the closest one, and it is scratchy. I'm located about 15 miles south of Chicago, and used to pick up both CFD channels great, now it comes in scratchy and pops in and out.
 

jackj

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From your description it sounds like you have a problem with the mount or coax. There used to be several types of NMO mounts, some used an extra washer that slipped under the center contact of the mount. This was used to help match the antenna depending on frequency. If it was used on an antenna that didn't require it then the antenna's performance would be degraded. Check the paper work that came with the mount to see if it is band/frequency specific.

All of the NMO mounts I have ever seen required a 3/4" mounting hole. Some would fit in a 3/8" hole but wouldn't give a good ground connection at the mount. The mount had a piece that fit inside the hole which would hold it centered in the hole and had sharp fingers that penetrated the paint and gave a good ground. Without a good ground at the antenna end of the coax you won't get a good ground plane for the antenna to work against and it won't work well. There's no reason why a NMO mount couldn't be designed for a 3/8" hole but I've never seen one that was.

Another thing to check is the coax. See if there is a part number or some type of nomenclature on the cable itself. If there is, Google it to find out the impedance and loss of the cable. I have some very good Teflon coax that is low loss. But it's 174 ohm and wouldn't work well for antenna feed line. Your feed line should have an impedance of 50 ohms and should have a loss of less than 10-12 db per 100'.

If all of this checks out then I'm at a loss as to what is causing your problem. Without actually seeing the installation it is very hard to trouble shoot.

Good luck
 
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